Eighty million years ago, tyrannosaurs were the top predators in Asia and North America.

And scientists say a newly discovered dinosaur from Uzbekistan helps to explain their rise.

In a paper published Monday in the Proceedings of the National Academy of Sciences, researchers said they have found a specimen from a 20 million-year gap in fossil records — between the small-bodied "marginal hunters" and the "apex predators" the tyrannosaurid group would become. This group includes Tyrannosaurus rex, Albertosaurus and Tarbosaurus.

As the paper explained, the rise of tyrannosaurs from the smaller, earlier species to "late Cretaceous giants was one of the seminal events in dinosaur evolution." But exactly how they developed has been mysterious, the scientists said:

"Little is known ... about how tyrannosaurids developed many of their signature features, such as their gigantic size, highly unusual brains, ears attuned to low-frequency sounds, and extensively pneumatized skulls."

The newly discovered dinosaur called Timurlengia euotica — which the scientists said is 90 million to 92 million years old — is horse-sized, compared to Cretaceous tyrannosaurids, which could reach more than 10 meters long and weigh more than 5 tons.

The new species "shows that tyrannosaurs had yet to achieve huge size at this time but had already evolved key brain and sensory features of the gigantic latest Cretaceous species," the researchers said.

The researchers added that Tyrannosaurs "apparently developed huge size rapidly during the late Cretaceous, and their success in the top predator role may have been enabled by their brain and keen sense that first evolved at a smaller body size."

And the scientists said the date of this specimen suggests the tyrannosaurs' growth to "huge size and ecological dominance" happened suddenly — though it's still a mystery what the trigger was.

This paper was prompted by the discovery of a well-preserved braincase of T. euotica in Uzbekistan's central Kyzylkum Desert, linking together other specimens. The braincase was particularly revealing because it provides evidence about the dinosaur's sensory abilities.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.